Field of the Invention
The invention relates to a turbine shaft of a steam turbine, in particular for the combined accommodation of high-pressure and intermediate-pressure blading. The invention also relates to a method of cooling a turbine shaft of a steam turbine.
The use of steam at relatively high pressures and temperatures helps to increase the efficiency of a steam turbine. In addition, the use of steam in such a steam state makes increased demands on the corresponding steam turbine. A combined high-pressure and intermediate-pressure turbine is suitable in the case of a steam turbine of the lower to medium power rating, for example from 300 MW up to 600 MW. In that case, both high-pressure moving blades and intermediate-pressure moving blades are accommodated by the turbine shaft. The turbine shaft is accommodated in a single casing which has the allocated guide blades. An advantage of a steam turbine in which the high-pressure and intermediate-pressure blading are disposed in a common casing is, for example, that despite a more complicated type of construction, there is a shorter overall length and no bearing is needed. The common casing may have an inner casing and an outer casing which in each case are split horizontally and bolted to one another. The live steam state characterized by the high-pressure steam can at present be around 170 bar and 540.degree. C. In connection with the increase in efficiency, a live-steam state of 270 bar and 600.degree. C. may be aimed for. The high-pressure steam can be fed in a middle region of the turbine shaft to the high-pressure blading and flows through the latter up to an exhaust connection. The steam which is thus expanded and cooled down may be fed to a boiler and heated up there again. The steam state at the end of the high-pressure part is referred to below as cold reheating and the steam state after leaving the boiler is referred to as hot reheating. The steam coming out of the boiler is fed to the intermediate-pressure blading. The steam state can be around 30 bar to 50 bar and 540.degree. C., in which case an increase to a steam state of about 50 bar to 60 bar and 600.degree. C. is aimed at. Further investigations would be required to determine the extent to which the materials used heretofore for manufacturing corresponding turbine shafts and turbine casings, in particular a chromium steel having 9% to 12% by weight of chromium, can meet the requirements at relatively high steam states. The moving blades in the steam-admission region of both the high-pressure part and the intermediate-pressure part may be manufactured from a nickel-based alloy. Furthermore, structural measures in which the turbine shaft is protected by shaft shields from direct contact with the steam may be carried out in the steam-admission region.
Published Japanese Patent Application 59 034 402 relates to a steam turbine with a hollow turbine shaft. Steam which flows into the turbine shaft serves to drive the turbine. The steam turbine is formed of a single partial turbine, in the middle region of which steam which is already partly expanded flows into the interior of the turbine shaft. The steam which flows at that location is divided by a throttle into two streams, namely into a cold partial flow which is conducted in the direction of the steam-admission region, and a hot partial flow which is conducted in the direction of the exhaust steam region.